Bolts or nuts have previously been provided with an extending flange which has a wavy bearing surface, including crests and troughs, the crests engaging a workpiece upon initial installation, and then upon tightening of the threaded fastener the troughs of the wavy surface are pulled into engagement with the working surface of the workpiece. When this occurs, it indicates that the threaded fastener has been stressed to a predetermined proper amount. Such a load indicating fastener is shown in U.S. Pat. No. 4,293,256. This may be utilized to establish a predetermined preload on the fastener, the fastener being subject to cyclic or vibrational forces which might otherwise tend to loosen the threaded fastener unless it was properly stressed. The subjecting of the threaded fastener to a varying load can cause fatigue failure; hence, the endurance, strength and fatigue life of the fastener are considerably improved by applying a precise preload to the fastener.
Torque wrenches or other torque measuring devices have been previously used to attempt to determine the preload on the fastener. Such methods are inaccurate because many factors affect the resistance encountered by a torque wrench, including type and degree of heat treatment, the type and surface finish of the threads on both threaded fasteners, and the coefficient of friction between the two threaded fasteners as well as between the fastener and the working surface of the workpiece.
It has also previously been known to utilize a washer with a threaded fastener as an indication of the tightness of such threaded fastener. A known load indicating washer was one which had on a first face thereof a series of dimples pressed therein so that on the opposite face a series of complementary projections were provided. These projections were about midway of the radial extent of the washer and when the washer and threaded fastener combination was properly tightened against the working surface, the projections in the washer flattened out completely so that even a thin gauge could not be inserted at any place around the periphery of the washer between the washer and the working surface, thus determining the tightness of the threaded fastener.
U.S. Pat. No. 4,281,699 illustrated another form of a load indicating flange on a threaded fastener wherein the flange was initially planar except for biting teeth and, when tightened, was then deflected into a wavy surface by buttresses which strengthened the flange intermediate the locking teeth.
With the wave type load indicating flanges of all of these prior art units, regardless of whether the flange was unitary with the threaded fastener or merely was a washer intended to be used with a threaded fastener, it was very difficult to be certain when the threaded fastener was tightened a proper amount. Where the wavy surface of the flange was supposed to be tightened flat against the working surface of the workpiece, the visibility might be poor for the inspector, so that the crests could not readily be distinguished from the troughs in order to determine where to attempt to insert the thin metal gauge. Further, since the troughs were supposed to be tightened flat to within 0.001 inch of the workpiece, the inspector was attempting to utilize a gauge having a thickness of only 0.001 inch in thickness, and such metal gauge was far too thin to have structural rigidity and therefore was easily damaged.